(1) Field of the Invention
The present invention relates to a coordinate input device which detects coordinates of an input image on a transparent writing panel by utilizing propagation of surface acoustic waves on the writing panel, and inputs the coordinates to an image display system so that the input image is displayed on a display screen of the image display system.
(2) Description of the Related Art
An image display system in which a coordinate input device is incorporated therein is known. In the image display system, an initial image is displayed on a display screen. For example, the coordinate input device has an image input panel, such as a touch screen, which covers a display screen of the image display system. The touch screen has a touch-sensitive transparent panel covering the display screen. The coordinate input device detects coordinates of points specified on the image input panel by an operator to form an input image, and inputs the coordinates to the image display system so that the input image and the initial image are displayed on the display screen in an overlaying manner.
Japanese Laid-Open Patent Application No. 8-305481 discloses a coordinate input device of the above type. The above-mentioned publication shows a background technology of the present invention. The coordinate input device of the above-mentioned publication utilizes propagation of surface acoustic waves (SAW) to detect coordinates of an input image on a transparent writing panel.
FIG. 7 shows a conventional coordinate input device as disclosed in the above-mentioned publication.
As shown in FIG. 7, the conventional coordinate input device has a transparent writing panel 1 covering a display screen of an image display system. The transparent writing panel 1 is made of glass and provided in the form of a rectangle having two vertical sides and two horizontal sides. Supposing that the writing panel 1 has an x-axis parallel to the horizontal sides and a y-axis parallel to the vertical sides, the position of a point on the writing panel 1 can be defined by a set of an x-coordinate and a y-coordinate. The x-coordinate and the y-coordinate of the point on the writing panel 1 are detected and input to the image display system by the conventional coordinate input device.
A transmitting transducer 2 and a receiving transducer 3 are provided at an upper left corner and a lower left corner of the writing panel 1 and both confront the right side of the writing panel 1. The transmitting transducer 2 transmits surface acoustic waves (SAW) in the horizontal right direction so as to propagate the SAW within the writing panel 1. The receiving transducer 3 receives the SAW from the transmitting transducer 2 through reflection.
A transmitting transducer 4 and a receiving transducer 5 are provided at a lower left corner and a lower right corner of the writing panel 1 and both confront the upper side of the writing panel 1. The transmitting transducer 4 transmits surface acoustic waves (SAW) in the vertical upper direction so as to propagate the SAW within the writing panel 1. The receiving transducer 5 receives the SAW from the transmitting transducer 4 through reflection.
In the conventional coordinate input device of FIG. 7, two arrays of reflectors 6 are provided on the horizontal sides of the writing panel 1, and the reflectors 6 are horizontally arrayed at a given angle on each horizontal side of the writing panel 1. Further, two arrays, of reflectors 7 are provided on the vertical sides of the writing panel 1, and the reflectors 7 are vertically arrayed at the given angle on each vertical side of the writing panel 1.
As shown in FIG. 7, the reflectors 6 on the upper side of the writing panel 1 partially reflect the SAW from the transmitting transducer 2 in the inward directions, and the reflectors 6 on the lower side of the writing board 1 further reflect the SAW from the upper-side reflectors 6 to the receiving transducer 3. The reflectors 7 on the left side of the writing panel 1 partially reflect the SAW from the transmitting transducer 4 in the inward directions, and the reflectors 7 on the right side of the writing panel 1 further reflect the SAW from the left-side reflectors 7 to the receiving transducer 5.
Further, in the conventional coordinate input device of FIG. 7, an input pen 8 is provided. The input pen 8 has a leading edge made of a rubber material. By pressing a specific position of the writing panel 1 with the leading edge of the input pen 8, an operator specifies a point on the writing panel 1. The SAW from the transmitting transducer 2 travel within the writing panel 1 in the y-axis direction through the reflection by one of the upper-side reflectors 6, and the receiving transducer receives the SAW from a corresponding one of the lower-side reflectors 6. The transmission and the reception of the SAW are repeated by the transducers 2 and 3 with respect to all of the reflectors 6 along the x-axis of the writing panel 1. At the same time, the SAW from the transmitting transducer 4 travel within the writing panel 1 in the x-axis direction through the reflection by one of the left-side reflectors 7, and the receiving transducer 5 receives the SAW from a corresponding one of the right-side reflectors 7. The transmission and the reception of the SAW are repeated by the transducers 4 and 5 with respect to all of the reflectors 7 along the y-axis of the writing panel 1. The propagation of the SAW within the writing panel 1 is cut off or inhibited only at the point specified by the input pen 8.
In the conventional coordinate input device of FIG. 7, the time which is taken for the SAW to travel from the transmitting transducer 2 to the receiving transducer 3 via the reflectors 6 in the writing panel 1 varies depending on the distance of each of the reflectors 6 from the left side (or the y-axis) of the writing panel 1 along the x-axis of the writing panel 1. By normalizing detection signals output by the receiving transducer 3, the distance of each of the reflectors 6 is converted into a variable in terms of time. The propagation of the SAW within the writing panel 1 is cut off or inhibited only at the point specified by the input pen 8. By locating a particular one of the reflectors 6 for which the detection signal output by the receiving transducer 3 is attenuated, the x-coordinate of the point on the writing panel 1 can be detected by the conventional coordinate input device of FIG. 7. In a similar manner, by using the transmitting transducer 4 and the receiving transducer 5, the y-coordinate of the point on the writing panel 1 can be detected.
In the conventional coordinate input device of FIG. 7, the surface acoustic waves (SAW) are periodically propagated in the x-axis direction and in the y-axis direction of the writing panel 1, in order to allow the detection of the coordinates of a point on the writing panel 1. If a writing panel with an increased size is used in the conventional coordinate input device of FIG. 7, the overall period of the propagation of the SAW may be excessively increased. It makes it difficult for the conventional coordinate input device to accurately detect the coordinates of a point specified on the increased-size writing panel by the input pen 8.
If the size of the writing panel used in the conventional coordinate input device is increased, the attenuation of the SAW propagated in the writing panel becomes considerable and the number of reflections or separations of the SAW by the reflectors 6 and 7 is increased. Hence, it is difficult for the conventional coordinate input device to use a writing panel having an increased size beyond a certain limit while allowing accurate detection of the coordinates of a point on the writing panel.
A conceivable method to eliminate the above problem of the conventional coordinate input device is that the ratio of the reflection or separation of the SAW by each of the reflectors 6 and 7 be reduced according to the increased size of the writing panel. However, when this method is used, the level of the detection signals output by the receiving transducer 3 or 5 becomes low. The SAW in one propagation path on the writing panel are easily influenced by the SAW in other propagation paths on the writing panel, and this makes the signal-to-noise (S/N) ratio lower. Hence, when the above-mentioned method is used, it is difficult for the conventional coordinate input device to use a writing panel having an increased size while allowing accurate detection of the coordinates of a point on the writing panel.